In many forms of electronic equipment, signals are communicated between groups of plug in modules in either analog and/or digital formats over a collection of electrical conductors commonly referred to as a backplane. Historically, this group of signals has consisted of N information carrying signals combined with a single return or ground signal. In these types of systems, the transmission performance is limited by a variety of cross talk mechanisms between conductors. For example, mutual inductance, mutual capacitance and ground noise or ground bounce all provide mechanisms for one signal to interfere with one or more other signals. Also, single ended conductors may be particularly susceptible to coupling of external noise sources such as interference from nearby radio transmitters.
FIG. 1, depicts a conventional legacy shelf. The legacy shelf or dual channel bank includes bank A having forty-eight line cards and bank B having forty-eight line cards. Six wires A1 through A6 connect the line cards in bank A to a common card C. Similarly, six wires B1 through B6 connect the lines card in bank B to the common card C. The line cards receive the signals with respect to ground. FIG. 2 depicts ground noise or ground bounce that occurs due to the configuration of the legacy shelf. In addition to ground noise or ground bounce, the signals on each of the six wires in banks A and B experience interference from the other wires in a given bank. This interference or inter-bank cross talk can be coupled onto the signals carried by each of the wires in a given bank as a result of parasitic circuit elements. These parasitic elements result from the physical implementation of a system. For example, two printed circuit traces running next to each other will have a certain amount of both parasitic capacitance and parasitic mutual inductance that allows the signal on one trace to interfere with the signal on the other trace and vice a versa. Typically, a given system is designed to limit the parasitics through careful choice of signal spacing, trace widths, board materials, etc. Since there is a cost associated with controlling the parasitics of a given system, the system is designed to only limit the parasitics just enough for a particular application. However, a significant problem arises when it is necessary for a system to meet a new application. For example, when it is necessary to have the system carry higher frequency signals and thus a higher data rate than that for which it was originally designed the system is ill equipped as the parasitic coupling increases with frequency. More specifically, the prior design simply does not adequately compensate for the increase in parasitic coupling due to the increase in frequency. One aspect of the present invention is designed to address this problem without substantially altering various aspects of the system, e.g., signal spacing, trace widths, board materials, etc.
In previously known systems utilizing higher signal bandwidths, differential transmission has been employed whereby a signal is transmitted on one half of a tightly coupled pair of conductors and the negative version of the signal is transmitted on the second half of the pair. This type of system is disadvantageous as nearly twice as many signal wires are required to transmit the same number of streams of information. This associated disadvantage is particularly exacerbated when it is desired to increase the capacity of an existing backplane. In this case, it is desirable for a solution to operate without any changes in the physical backplane, even when such changes could very well improve the transmission capacity.
In another known approach, highly advanced signal processing techniques referred to as multi-user detection and iterative interference cancellation may be employed to separate the signals. These techniques are complex and require large amounts of computation to separate the signals. Thus, this approach may be unacceptable in applications where tight cost or power budgets must be met.